The present invention relates to air spray guns, and in particular to an improved fluid valve stem for an air spray gun that allows effective pneumatic atomization of coating material to be achieved at low air pressures and volumes for increased transfer efficiency.
To decrease the cost of coating material used in spray coating processes and for environmental considerations, there has been a trend toward spray coating equipment having a high transfer efficiency. Transfer efficiency is the amount of coating solids applied onto a target versus the amount of coating solids sprayed, expressed as a percentage. To increase transfer efficiency, the velocity of the coating particles should advantageously be fairly slow in order to avoid blow-by which occurs when spray particles miss the target, with excessive velocity of the particles actually causing some of them that strike the target to bounce off of it. Greatest transfer efficiency is usually achieved in systems offering optimum atomization coupled with the lowest possible velocity of the spray particles.
Conventional air spray guns have a relatively low transfer efficiency. Air delivered to their spray heads has a relatively high pressure and volume, and as it exits the spray head it atomizes a solid cylindrical stream of liquid coating material into a conically-shaped spray, which usually is flattened into a fan-shaped pattern by opposed side port air jets. When the high pressure and high volume air exits the spray head, it expands and imparts a relatively high velocity and fogging effect to the spray particles, causing a large percentage of the particles to miss the target.
Airless spray systems have a somewhat higher transfer efficiency. With such systems, coating liquid is hydraulically forced through a specially shaped orifice at pressures on the order of 500-4500 psi, which causes the coating to be emitted in an unstable thin film that interacts with atmospheric air and breaks up into an atomized spray at its forward edge. These systems develop spray particles that have a lower velocity and exhibit less fogging than occurs with conventional air spray guns.
A more recent development is the air-assisted airless system which utilizes both airless and air atomization. Coating liquid is supplied to a specially shaped orifice at hydraulic pressures less than those normally encountered in purely airless systems, usually on the order of 300-1,000 psi. This causes the material to be atomized into a spray, but the degree of atomization is not as satisfactory as that obtained with conventional airless or air spray guns. To improve atomization, an air-assist is applied to the spray pattern, enhancing the atomization process and doing away with tails that would mar the finish. The transfer efficiency of air-assisted airless systems is greater than those of conventional airless or air spray systems.
Recently, high volume low pressure (HVLP) spray systems have found increasing use because of their high transfer efficiency. These systems utilize air to atomize a stream of coating material, but at the spray head the air has a relatively high flow rate, usually well in excess of 5 CFM, and a relatively low delivery pressure, usually less than 15 psi. The high volume and low pressure of the air results in decreased fogging and an increased percentage of the spray particles striking and adhering to the target.
All of the mentioned spray coating systems have certain disadvantages. Conventional air spray guns have a relatively low transfer efficiency, and although airless and air-assisted airless systems have a higher transfer efficiency, they require separate pumps for delivering liquid coating at the pressures required, which adds to the cost and complexity of the systems. In the case of HVLP spray guns, many require use of a separate turbine to supply air at high volume and low pressure to an inlet to the gun, while others adapted for use with high pressure factory air are usually provided with specially configured and complex air flow paths to ensure that the pressure of air at their spray heads does not exceed a selected maximum value.